Methods for the formation of black films or coatings on the surfaces of various metal are currently available. The methods vary with the particular type of metal, e.g., ferrous metals, stainless steels, zinc and its alloys, aluminum and its alloys, copper and its alloys. The composition of the treatment solution and the treatment conditions vary from case to case.
Japanese Patent Publication No. 56-33155 (33155/81)in the name of Nihon Parkerising Company, Ltd. discloses a method of forming a black coating with an aqueous resin containing solution followed by baking to produce a coating of the desired weight. The aqueous solution contains a hexavalent chromium compound, a reducing agent and water soluble resin.
In the treating of many aluminum or aluminum alloy articles the more common procedure is to first apply a conversion coating such as a chromium based coating and thereafter to apply a solvent based black paint. The first step creates a corrosion resistant conversion coating. The second painting step is almost exclusively a decorative and aesthetic step. Such two step processes require extra equipment and waste disposal procedures. Coatings such as described in Japanese Patent Publication Number 56-33155 also require ovens to bake the coated metal to form the coating thereon. Similarly, U.S. Pat. No. 4,931,317 discloses a method and composition for the formation of a black film or coating on the surface of various materials by coating and subsequent baking of a treatment solution containing ferrous metal ions, hexavalent chromium, trivalent chromium and a film forming polymer dissolved or dispersed in water. The black nature of the coating is due to the concentration of hexavalent chromium and metal salts (Co, Fe and Ni) in the solution.